I definitely think a reconsideration of her participation in the wedding party is in order.
Sometimes, groups of friends have a very specific social heirarchy and dynamic that has become so ingrained that it's taken as gospel. Then, when late in the game the dynamic is changed, other people who have relied on their position in the heirarchy become very threatened and the results are somewhat akin to what is happening here.
By way of background, when my husband and I married, he was part of a group of people in our shared home town who had known each other, directly and by extension, for a number of years, since high school. At the time that we married, they were all in their mid to late 30's. DH was a late bloomer in life. All of his "crowd" were married and many with kids, and there was only one single person, a woman, left in the circle. DH was still figuring out where he fit professionally, having wanted to be a teacher for many years but, unable to afford college and needing to earn a living, was slogging it out one-class-at-a-time. Meanwhile, the other singleton in the group, the female, was a teacher already.
When DH and I met and he told me his dreams, I said "go for it. I make enough money for both of us." He quit his dead-end job, went back to school full time, received multiple honors and graduated with highest marks, and walked out of school into a teaching position in a private school. During that interim, he and I married. So he went from being the "poor guy, never got his act together and is still alone" to achieving to his highest abilities and getting married. In other words, he left his spot at the bottom of the heirarchy.
The woman in his group who was still single did exactly to me what BW is doing to you - attempting to erode my faith in my sweetie. It worked exactly once. The second time she tried it, my response was "I can't imagine why you think he wouldn't succeed." (She had told me that, based on her experience, that DH was a "bad bet.") After that, she would literally get up and leave any gathering when DH and I arrived. The audacity of DH! To upset the social order!
And so that is, IMO, what is happening here. She feels unsteady in her place in the heirarchy, although I doubt that she can recognize it. And since you were the catalyst for the change, her attempts to restore the prior order must begin and end with you.
Drop her from the bridal party - she does not truly wish you and your fiance well, and that is the primary requirement for a member of the wedding party.